A research team at Florida State University (FSU) recently announced that they have discovered a new perovskite material that can be used in photovoltaic cells for capturing and converting infrared light or radiation. Most crystalline silicon photovoltaic cells that are now on the market cannot convert infrared light into electricity because infrared rays have longer wavelengths and carry less photon energy. The material technology pioneered by researchers at FSU could lead to a huge improvement in the conversion efficiency rate of photovoltaic cells as it would allow the utilization of a significant portion of the solar spectrum that was previously unavailable for the conversion process.

The visible part of the sunlight only constitutes a portion of the electromagnetic radiation given off by the Sun and absorbed by the Earth. Ultraviolet and infrared radiation, for instance, are two classes of radiation that are beyond the visible light range in the electromagnetic spectrum. The amount of photon energy that drives photovoltaic cells is inversely proportional to the wavelength of the light or radiation. The shorter the wavelength, the higher level of photon energy. Conversely, infrared radiation with wavelengths longer than those of visible light does not have sufficient energy to excite the electrons in the semiconductor material of a typical photovoltaic cell.

The lack of capacity to utilize the entire solar spectrum means that today’s photovoltaic cells still have a lot of room for improvement with respect to efficiency. When the sunlight hits a working solar panel, the panel will be able convert the near-infrared light that is within the edge of the visible light range and has just enough energy to excite the electrons within the cells. However, the rest of infrared radiation that the panel has absorbed simply becomes heat. As for ultraviolet radiation and visible light, the solar panel will be able to turn them into electricity because of their short wavelengths.

To improve the conversion efficiency rate, the FSU team has devised an approach known as “photon upconversion.”

The idea behind this approach is to optimize the surface of a photovoltaic cell so that when two or more low-energy photons from the long-wavelength infrared light hit the cell, they will convert into one high-energy photon that emits visible light. Previously, scientists at Lawrence Berkeley National Laboratory created an organic dye that can absorb infrared radiation and re-emit it as visible light.

Lea Nienhaus, who is an assistant professor of chemistry and biochemistry at FSU and a member of the research team, said that her team’s idea in part draws from the earlier study done at Berkley Lab. The main point of differentiation is the material. The FSU team has developed a thin film consisting of lead-halide perovskite and a hydrocarbon compound called rubrene for the upconversion process.

Finding the right thickness for the thin-film material is the key to raising the conversion efficiency rate. The FSU team has run trials on films that are 20, 30, 100, and 380 nanometers thick. The results from their tests indicate significant rate improvement when the thickness is greater than 30 nanometers.

Despite its incredible capacity to change infrared light into visible light, this perovskite film also has a noticeable drawback. Sarah Wieghold, a postdoctoral researcher working with Nienhaus, said that the material will re-absorb some of the visible light it produces during the upconversion process. Hence, the FSU team still has to further engineer the perovskite cell so that it achieves the optimal ratio of the infrared light input versus the visible light output.

The study on the properties and application of this particular perovskite material was published in the academic journals Matter and the Journal of Physical Chemistry Letters.